A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to monitor the lithographic process, it is desirable to measure parameters of the patterned substrate, for example the overlay error between successive layers formed in or on it. There are various techniques for making measurements of the microscopic structures formed in lithographic processes, including the use of scanning electron microscopes and various specialized metrology tools. One form of specialized metrology tool is a scatterometer in which a beam of radiation is directed onto a target on the surface of the substrate and properties of the scattered or reflected beam are measured. By comparing the properties of the beam before and after it has been reflected or scattered by the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties. Two main types of scatterometer are known. Spectroscopic scatterometers direct a broadband radiation beam onto the substrate and measure the spectrum (intensity as a function of wavelength) of the radiation scattered into a particular narrow angular range. Angularly resolved scatterometers use a monochromatic radiation beam and measure the intensity of the scattered radiation as a function of angle.
A metrology tool such a scatterometer typically includes a base frame and a substrate stage constructed to carry a substrate movably connected in the “Z” direction with respect to the base frame, using a first displacement system. The substrate stage is typically also provided with a second displacement system configured to rotate the substrate table in the X-Y plane with respect to the substrate stage. Above the substrate stage along the Z direction, there is provided a sensor stage arranged to detect radiation scattered from the substrate, the sensor stage being movably connected to the base frame by means of a third displacement system. Each displacement system may be fixed to the surrounding frame. However this may introduce vibrations in the frame, which may limit performance and/or throughput. In order to minimize this, it is known to provide at each stage, that is the substrate stage and the sensor stage, a balance mass designed to avoid acceleration forces from being applied to the frame.
EP 1 862 856 discloses a metrology tool arranged to measure a parameter of the substrate, that has been provided with a pattern by a lithographic apparatus. The metrology tool includes a base frame, a substrate table constructed and arranged to hold a substrate, and a sensor arranged to measure a parameter of the substrate. A displacement system is provided to displace one of the substrate table and the sensor with respect to the other in at least one direction. A balance mass is provided, a first bearing movably supporting the balance mass so as to be substantially free to translate in the opposite direction to the displacement of the substrate table or the sensor. The substrate stage in the balance mass may form part of an integrated linear motor, in which the wafer stage is provided with a stage carrying coils acting as a rotor, while the balance mass is formed as a plate acting as an armature, both armature and stator being guided with respect to the base frame. The balance mass is provided at a position displaced horizontally with respect to the base frame, bearing directly onto the base frame. Such an arrangement may however, in a metrology tool in which is space is very limited, increase the footprint of the machine in order to accommodate the balance mass.